An electron source for use in such a particle-optical apparatus is known from European Patent EP 0 366 851.
A variety of electron sources are commonly known for use in particle-optical apparatus. Known are thermal emitters of metal or metal oxides, field emission sources, semiconductor emitters, Schottky emitters and Metal-Isolator-Metal (MIM) cathodes. For use in particle-optical apparatus (for example electron microscopes), generally speaking source properties such as high brightness, constant beam current, small energy spread of the electrons in the emitted beam, mechanical ruggedness of the electron source and simplicity of construction are desired. Depending on the field of application of the electron-optical apparatus, different ones of these properties will be emphasized.
The cited EP Patent document describes an electron source in which a tungsten needle is arranged opposite an extraction-electrode of a thickness of less than 1 .mu.m. The tungsten needle is shaped so that it has a tip of atomic sharpness, i.e. its tip is formed by a single atom or by a group of, for example three atoms ("trimer"). The extraction-electrode is provided with a round aperture whose diameter is of the order of magnitude of the thickness of this electrode. Elsewhere in the cited Patent document it is stated that the distance between the tip and the aperture is of the order of magnitude of the diameter of the aperture whereas this distance is between 1 and 10 nanometers. No further indication as regards the size of the aperture can be derived from the cited Patent Document.
As is known, the brightness of an electron source (at constant current and emission space angle) is inversely proportional to the magnitude of the emitting surface, so that this electron source enables a high brightness to be achieved because of the sharpness of the tip of its needle.
The sharpness of the tip of the needle is of essential importance in the known electron source. However, much effort is required to provide the needle with such a sharp tip. Moreover, the needle must be arranged accurately opposite the centre line of the aperture in the extraction-electrode and the distance between the needle and the extraction-electrode also has to be monitored constantly. Finally, in this known configuration special steps will be required to prevent or compensate for changes (for example, due to thermal effects) in this alignment.
The aperture in the extraction-electrode of the known electron source must be very accurately circular, because otherwise astigmatism or other deformation of the electron beam occurs; this is inadmissible for practically all applications of such an electron source. However, it is objectionable to form an aperture of such a diameter (i.e. 1 .mu.m or of the order of magnitude of from 1 to 10 nm) so as to be round with the required accuracy.